Electron gun



D. F. MOONEY June 6, 1961 ELECTRON GUN Filed Dec. 11, 1959 OO/VALD E MOO/Viy INVENTOR.

BY M /r m H/S ATTORNEY energy requirements or to other causes.

2,987,648. ELECTRON GUN Donald F. Mooney, 1311 NW. 8th St., Gainesvllle, Fla. Filed Dec. 11, 1959, Ser. No. 858,998 Claims. (Cl. 315-14) The present invention relates to an electron gun and more particularly, to an electron gun capable of high voltage pulsed beam generation.

Devices for producing electrons and accelerating them to high velocities are called electron guns. They are the electron sources in television tubes and many other electronic devices such as klystrons, traveling wave tubes, and linear accelerators. The present invention relates to an improved electron gun.

Accordingly, an object of the present invention is to provide an improved electron gun.

In some electron gun applications the electrons must be emitted for very short periods of the order of a few microseconds. This is often due to high acceleration Additionally, the beam often must be accelerated to a high voltage. These two features of short beam duration and high beam voltage have ben obtained, in prior electron guns, through modulation of the high voltage that accelerates the electrons, which voltage may be as great as 80 kilovolts. In other words, it has been necessary to have sources for producing high voltage pulses of extremely short duration. It is apparent that such sources present many problems in construction and cost.

Thus, another object of the present invention is to provide an electron gun for producing a high voltage electron beam, the magnitude of which can be controlled by the utilization of a relatively low voltage.

In many applications, and particularly in linear electron accelerators and traveling wave tubes, best results are obtained with a high density, small diameter beam. This beam is usually produced through convergence of a much larger diameter beam by means ofa complex structure that with the electron gun usually requires 5 or 6 electrodes.

Therefore, another object is to provide an incomplex electron gun having a minimum number of electrodes for producing a pulsed, high voltage, and convergent electron beam.

These and other objects are achieved in a preferred embodiment of the present invention in which the electron gun comprises a cathode electrode section, a grid electrode, and first and second anode electrodes. In the cathode electrode section, two coaxial cylinders are connected to a flat-surfaced electrode that extends out from and even with the cathode electrode. These cylinders and the fiat electrode coact with the first anode electrode to produce the potential distribution required for parallel electron flow between the cathode and first anode electrodes. The grid electrode, which is fiat, is mounted in the region of a planar equipotential and during the flow of current is maintained at this potential so as to produce as little disturbance as possible. At other times, it is maintained at a relatively low potential to prevent the flow of electrons. The first anode electrode is energized with a small portion of the acceleration potential. The remainder of this potential is applied to the second -anode electrode where it accelerates the electrons and also causes them to converge to the desired diameter.

7 The novel features believed characterisic of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference to the following description, taken in connection with the accompanying drawing, in which:

2,987,648 Patented June 6, 1961 FIGURE 1 is an elevation view of a preferred embodiment of my invention, and

FIGURE 2 is a cross-sectional view taken along the line 2--2 in FIGURE 1.

Referring now more particularly to FIGURE 1, in which a preferred embodiment of my invention is illustrated, there is an electron gun comprising a cathode electrode section, a grid electrode 12, a first anode electrode 13, and a second anode electrode 14.

In the cathode section the electrons are emitted from the flat surface 16 of a cathode electrode 17 that is heated by a heating element 18. Surface 16 is preferably coated with material capable, upon heating, of emitting large numbers of electrons per unit of time; i.e., with material having a low work function. Around cathode electrode 17 a gap 19 or well provides thermal insulation for minimizing heat losses.

Cathode electrode 17 is surrounded by a flat-surfaced electrode 20 which is even with and electrically connected to cathode electrode 17. On electrode 20 there are two cylinders 22 and 23, the inner one (22) of which is of lesser height than the other (23), and both of which are coaxial with cathode electrode 17 to provide a symmetrical arrangement about the electron beam. For optimum operation, the diameter of electrode 20 as well as the heights and diameters of these cylinders have interdependent vlaues, as will be explained later.

The positive potential difference between anode electrode 13 and cathode electrode 17, required for electron flow, is preferably obtained by maintaining the cathode potential at the full acceleration potential below ground and the anode electrode 13 several thousand volts more positive, but still negative with respect to ground. Second anode electrode 14 is preferably at ground potential.

The potential difference between cathode electrode 17 and anode electrode 13 can be analyzed into a plurality of non-intersecting diiferent equipotential surfaces, extending, roughly speaking, parallel to cathode electrode 17 and anode electrode 13. Actually, the equipotentials are planar in only a narrow region, but this region is sufficiently large to permit the insertion of a fiat grid electrode 12, which may include a mesh 25, without significantly disturbing the potential distribution existing during current flow. Thus, due to the planar equipotentials, the grid electrode 12 may be flat, which is important since a flat electrode is much easier to construct than the complex, curved electrodes required in prior electron guns.

In the above-mentioned potential distribution, the region of planar equipotentials is several thousandths of an inch wide, which is sufficiently wide such that the grid electrode 13 may be relatively thick and, as aresult, structurally strong. This strength prevents warping when grid electrode 12 becomes hot due to bombardment by some of the electrons. Warping would cause a change in grid shape and thus a change in the electric field from that desired.

The first anode electrode 13 has a flat surface facing grid electrode 12. Also, it has an aperture 27 that may be covered by a mesh (not illustrated).

Around the periphery of first anode electrode 13 is an insulating region illustrated as an air gap, that is interrupted at a few places to provide supporting tabs 31 (shown in FIGURE 2). Due to this region, a better field configuration is obtained.

Second anode electrode 14 is illustrated as a separate structure utilized only for accelerating and converging the electrons. Alternatively, it may be the foremost part of the utilization structure, which, for example, may be a buncher section.

The electrodes are supported by insulating cylindrical elements 33, 34, and 35, that may, for example, be comprised of a ceramic material.

comparatively low potential gradient between first anode electrode 13 and cathode electrode 17 followed by the comparatively high potential gradient between second anode electrode 14 and first anode electrode 13, produces a thin lens action causing the electron beam to converge. The spacings and necessary potential relationships for this thin lens action are well known'in the art and are described in readily available texts, as for example:

' Vacuum Tubes, Spangenberg, R.,' McGraW-Hill" Book (2b., Inc., New York, 1948.

The converging action continues until it is counterbalanced by the mutual repulsion of'th'e' electrons. At this time, the electron beam is at a minimum diameter and thus is then most suitable for injection into a utilization section, e.g., a linear electron accelerator, or some other type structure, the particular structure depending upon the particular application.

The illustrated lens system is preferred since it coacts with the electron gun to produce a convergence with the minimum number of electrodes. However, if desired, other lens systems can be employed.

The electron beam, although convergent after leaving anode 13, has parallel flow between cathode electrode 12 and anode electrode 13; i.e.,the electron trajectories are parallel. This flow can be explained by the principles of conduction in a space charge limited diode which comprises parallel planes of infinite area. In this theoretical arrangement the potential variation along the direction of electron flow, which is parallel flow, is as the four-thirds power of the distance from the cathode, or mathematically: Y

Z 2 'V=5680J X wherein V is the potential at a distance 'X from the cathode, and I is the current density in amperes per unit area.

This potentialvariation would at first appear to preclude practical considerations since it is' obtained between two parallel plates of infinite area. However, al-

though such sized plates are not practicaL'parallel' flow can be produced if the potential distribution at the edge of'the electron beam is the same as that'obtained with electrodes'of infinite area. In other words, b'y'creating an environment at the edge of the electron -b'earn that is the same as that occurring in the theoretical case, an electron beam action can be obtained which is the same as that of the theoretical case. In addition to the fourthirds distribution, the potential gradient normal to the beam should be zero to prevent beam spreading.

. positioned after anode electrode image, and therefore converges I have discovered that this potential distribution can I be obtained, to a satisfactory degree, by the utilization of at least two cylindrical electrodes 22 and 23 acting conjunctionally with plate electrode 20 and anode electrode 13. Two cylindrical electrodes provide satisfactory results for most applications, but more can be used if mounted coaxially.

Suitable dimensions for cylinders 22 and 23 and plate 20 can be obtained from an electrolytic tank. The tank need be used only once, for after a set of dimensions has been found, other suitable dimensions for different structures and different electron beam densities and voltages can be calculated by scaling the obtained values in the manner described in the article: Engineering Methods in the Design of the Cathode Ray Tube, Moss, 11., British Institution of Radio Engineers, p. 204, vol. '5,

The use of the tank is conventional. Itis tipped until grid electrode 12,

. tothe potential on the the high resistivity electrolyte in the tank meets the tank bottom along a line. The electrolyte is then wedgeshaped indepth. This wedge represents a wedgeshaped section of the electron beam with the meeting line corresponding to the beam axis. An elongated piece of dielectric is then placed in the tank parallel to this meeting line and at a distance from itso that the wedgeshaped electrolyte between this line and the dielectric corresponds to the aforementioned beam section. With this arrangement, the electric field at the boundary of the dielectric is parallel to the dielectric surface. The insulating properties of the dielectric prevent the charge distribution necessary for a radial field component. As has been mentioned, the parallel electric field is'desired because a radial comp'onent'would cause thebeam to spread. Thus, one field condition is obtained merely by inserting the piec'e'of dielectric into the'tank; To obtain the other'ficld distribution, namely, potential distribution in accordance with the previously mentioned fourthirds law, a plate electrode is placed at one end of .the dielectric to represent anode electrode 13 and thecathode section 11 is placed at the other end. Then the dimensions of the cathode section'11, i.e., the heights of cylinders '22 and 23 as well as their diameters .and that of plate 20, are varied until the desired potential distribution is obtained.

A more detailed explanation ofthe use of an electrolytic tank can be found in standard texts such as,for example, the aforementioned Vacuuin Tubes."

lmrnediatelyfollowing is a set of suitable dimensions for an electron gun in which anode electrode 13 is 2,000 volts more positive than cathode 17 and 'in which the beam density is .079 ampere per cm. During beam current vflow, ,grid electrode 12 is 660 volts above .the

potentialof cathode electrode 17, and is' at cathode electrode potential when no beam current is desired.

Dimension in inches (11) Width of annular portion of electrode 13 0.272

-With the four-thirds law potential distribution between the cathode .section' ll and, the first anode electrode "13, parallel .flow is obtained up to the first anode electrode aperture 27. The beam is thus well defined at the exit of aperture 27 and,as a consequence, any lens system '13 has a clearly defined the beam .very readily to the predetermined diameter. 7

As mentioned, the beam convergence can be produced by many difierent electron lens systems, but the one illustrated is preferred since, among other advantages, it requires so few elements.

Another advantage of the present electron gun relates second anode electrode'14. Due .to the shielding actionof the first anode electrode 13 and I V 'which action is enhanced by meshes, this potential can be changed within relatively large limits without significantly changing the beam dimensions and current. 'Inother words, the electron beam magnitude and diameter are determined almost entirely by .the potential on the first anode electrode 13. As a resi 1lt, the beam velocity necessarytor any particular application can be obtained through control of the potential ,on electrode 1'4, and it is not necessary toredesign the'electron gun for'each different application.

aeezaae The fiat electrodes are also an important feature of the present invention. Many prior electron guns require curved electrodes, some of which are spherical portions. The advantage in the construction of flat electrodes as compared to these complex curved electrodes is apparent. It is also to be noted that only a minimum number of electrodes are required.

Another advantage is beam control with a relatively low potential. In the past, the beam has been controlled through modulation of the plate potential which is usually quite large. The control of this potential, in times measured in microseconds, presents many problems. However, with the present invention the plate potential may be steady and the beam controlled by a relatively low potential applied to the grid electrode 12. This lessened requirement permits a much simpler modulating structure.

The foregoing discussion has been directed to the operation of the electron gun in an on-off fashion. But, if it is desired to modulate the magnitude of the electron beam in a continuous fashion instead, this can be accomplished by applying the modulating potential to the grid electrode 13.

While my invention has been described with reference to a particular embodiment thereof, it will be understood that numerous modifications may be made by those skilled in the art without departing from the invention. I therefore aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electron gun comprising a cathode electrode with a fiat surface for producing an electron beam, a fiat electrode extending out from said cathode electrode in a direction parallel to said cathode electrode and even with it, means for electrically connecting said flat electrode to said cathode electrode, a first cylindrically-shaped conducting electrode mounted on said fiat electrode coaxially with said cathode electrode, a second cylindrically-shaped conducting electrode mounted on said flat electrode coaxially with said cathode and having a greater height and diameter than said first electrode, the heights and diameters of said first and second cylindrical electrodes being such as to establish a potential distribution along the edge of said beam in agreement with the formula V=568OJ X wherein V is the voltage at a distance X from said cathode electrode and J is the current density of said electron beam, and means for accelerating said electron beam.

2. An electron gun comprising a cathode electrode having a fiat circular electron emitting surface for producing an electron beam, an anode electrode having a flat surface spaced from and parallel to said cathode electrode, a fiat-surfaced electrode surrounding said cathode electrode and even therewith, means for electrically connecting said fiat electrode to said cathode electrode, a first cylindrically-shaped conducting electrode mounted on said fiat electrode coaxially with said cathode electrode to extend partially between said flat electrode and said anode electrode, and a second cylindrically-shaped conducting electrode of greater diameter than said first cylindricallyshaped conducting electrode and mounted on said flat electrode coaxially with said cathode electrode to extend partially between said flat electrode and said anode electrode, the height of said second cylindrically-shaped conducting electrode being greater than that of said first cylindrically-shaped conducting electrode, the heights and diameters of said conducting electrodes and the diameter of said flat electrode being such as to produce a potential distribution at the edge of said electron beam in agreement with the formula: V=5680] X wherein V is the voltage at a distance X from said cathode electrode and I is the current density of said electron beam.

3. An electronic device for producing parallel flow of an electron beam, comprising a flat-surfaced circularshaped cathode electrode for producing an electron beam, a circular-shaped flat-surfaced electrode extending out from and even with said cathode electrode, an anode electrode mounted parallel to and in spaced fashion from said cathode electrode, a first cylindrically-shaped conducting electrode mounted on said flat electrode coaxially with said cathode electrode to extend partially between said fiat electrode and said anode electrode, and a second cylinrically-shaped conducting electrode of greater diameter and height than said first cylindrically-shaped conducting electrode and mounted on said fiat electrode coaxially with said cathode electrode to extend partially between said flat electrode and said anode electrode, the heights and diameters of said cylindrically-shaped electrodes being such as to produce a potential distribution at the edge of said electron beam in agreement with the formula: V=5680J -"X wherein V is the voltage at a distance X from said cathode electrode and I is the current density of said electron beam.

4. The device as defined in claim 3 and a flat electrode mounted between said cathode electrode and said anode electrode in a region in which the potential distribution produced by potentials applied between said cathode and anode electrodes is substantially planar.

5. An telectron gun comprising a cathode electrode for producing an electron beam, a fiat electrode extending out from and even with said cathode electrode, an anode electrode spaced from said cathode electrode and parallel thereto, a plurality of cylinders mounted coaxially on said flat electrode and electrically connected to said cathode electrode, said cylinders having diameters and heights to produce a potential distribution at the edge of said electron beam in agreement with the formula:

wherein V is the voltage at a distance X from said cathode electrode and J is the current density of said electron beam, and a flat electrode mounted between said cathode electrode and said anode electrode in a region in which the potential distribution produced by potentials applied between said cathode and anode electrodes is substantially planar.

References Cited in the file of this patent UNITED STATES PATENTS 2,864,965 Wang Dec. 16, 1958 

